This part is actually pretty easy. Not rocket science at all. And it pretty much follows the same instructions as a V8 Miata. Waa Hoo! Something easy! If you’re using a Miata drive train, it’s really simple. Just bolt in the Miata sub frame assembly and you’re done.
But if you’re racing with a V8, that Miata diff won’t last long. You’ll need to upgrade to something stronger. There are a few choices here. They all have their plusses and minuses. So, once again, Choose wisely Weed Hopper.
The GM Cadillac CTS differential fits really well. But, it has had some durability issues. The fluid volume is a little bit low – which matters for cooling. But it is small and light weight. It’s easy to mount, and exhaust tubing fits around it really well. It’s a very popular choice.
You can use the Ford 7.5″ IRS differential. For a street car, I’m sure that’s fine. The Exocet only weighs about 1800 pounds. But for road racing it’s not strong enough – IMO – and has a such a low fluid volume that overheating will be a problem.
I used the Ford 8.8″ diff in a cast iron housing. The aluminum housing is a bit lighter, but it’s also bigger, and not as strong. They get broken in a high powered Cobra. Parts are readily available for the 8.8″ differential, and it’s just about bullet proof. You can beat on that for a lot of years before it fails. Durability and reliability are key points here. And, I already had one sitting on the shelf. 😉 Noticed that I drilled and tapped a hole in the bottom of the case for a drain plug.
This diff has 3.55 gears, and a Eaton Gear Tru-Trak LSD. The Tru-Trak is a modified Gleason worm gear, and does not have clutch packs. It is not a locker. It works very well on the road course. I ended up putting this particular housing in the Cobra, and another one built the same in the Exocet.
Getting it mounted to the Miata sub frame is tedious and time consuming. Take your time here and get it just right. If you don’t you’ll have a real hard time getting the pinion angle right. Measure 42 times, weld once.
Once you get it where you want, you can install it semi permanently into the sub frame. The subframe assembly can then be bolted into the chassis fairly easily. I have not added the lower braces yet. It’s much easier to bolt the diff to the sub frame, and then bolt the assembly into the chassis.
I used a motorcycle jack to lift the subframe, and slide it in to place. It’s only held in by 4 bolts at the top. That’s why the lower braces are so important for chassis stability and preventing wheel hop. I’ll get to those later.
Subframe Bracing
Updated 2/17/16
The stock Miata has a tubular brace that bolts to the four tabs at the bottom of the subframe, extends forward, and then attaches to the bottom of the car. You can find lots of pictures of that on the interweb. I find E-Bay to be an excellent source for parts pictures like that.
That would kinda work here. But then you have to find a place to mount the forward arms to the bottom of the car. Too complicated, IMO. And I wanted to keep the bottom of the car fairly flat, for protection during those off track farming excursions. Hey, it happens. 🙂
So, I started by making these steel tabs. Nothing fancy here, just something to attach the forward mounting points to the chassis.
Then, I needed to connect the four points together. In the spirit of adding lightness, I used some heavy wall 3/4″ aluminum square tubing. I did not weld them together, simply because I don’t have the tools to weld aluminum. Maybe some day. Then I can get rid of some fasteners.
On the right side, I had to trim the support bracket a smidgen. Maybe 3/8″, to allow the aluminum tube to sit flat. Not a big deal, couple of minutes with a cut off wheel.
Notice that the two side braces are not parallel. There’s a reason for that. If they are parallel, it creates a rectangular box, like the stock Miata brace. When they’re angled in like this, it creates a triangle. Triangles are much stronger than rectangles.
Is this going to be strong enough? Not sure yet. They didn’t bend or distort on the dyno that I could see. I’ll give them a closer inspection before I paint them. And I’ll inspect them after every race week end. If they bend or distort, I’ll move up to a 1″ square tube.
If I wanted to get fancy, I could put a bunch of dimpled holes in this tubing to make it stiffer, like the bottom of the lower A-arms. Not sure if that would work, but it would look cool. It might also create stress risers and cracks.
I suppose if I really wanted to get OCD about it, I would extend some bracing from from the bottom of the subframe up to the rear of the chassis, underneath the rear bumper. At the moment, I can’t see that it would provide any added value. But I’ll think about it.
2/22/16
I had the car on the Dyno last week. Made a number of hard runs to get the tune right. I took the braces out for inspection. The look pretty good, still straight and without distortion. They might actually work as is! But the Dyno isn’t the track. I’ll keep a close watch on them.
I took them out for painting and then installed with lock-tite in the frame nuts and nylock nuts on the 12mm through bolts. Yeah, I know, overkill. But it makes me feel better.
I labeled it “No Strap”, because it’s not strong enough for a tow strap or a jack. “19” is the size of the socket needed there. I’v done that throughout the car so I don’t have to guess what size wrench I need.
Parts Breakage, 9/9/16
This parts failure is clearly my fault. I installed the front bushings incorrectly, and they failed. They are supposed to have a large washer top and bottom. And I did not do that. I incorrectly assumed the large round base on the front mounts would support the bushing. It took a lot of track time for this to fail. Oh, well, live and learn.
That allowed the front of the bushing to flop around, and caused the rear brace to fracture. Notice that it cracked right through the “V”, an obvious weak point.
I could get out the welder and repair this; it wouldn’t be that hard to do. But I’m just going to buy a new one and be done with it. I asked V8 Roadster for one without the logo, since that’s an obvious weak sot. But they didn’t make them that way. They said they could custom make me one, but it would take them 4-5 months, when they made the next batch.
I considered using these aluminum bushings in the front, instead of the poly. My concern is that the rear brace uses poly bushings where the attach to the cradle. Aluminum in the front and poly in the back may create enough binding, and cause the ears on the differential to break. I’v seen that on high powered Superformance Cobra’s. So I’m just going to use some new Poly bushings. Maybe not as good for racing, but probably more durable.
7/2021
Well, here we are again. The differential brace is broken again. I could just order a new one. But in a few seasons I’d be here again. Time for a permanent fix. My main goal with car is reliability. You can’t win if you can’t finish.
The original part is made from form stamped 1/8″ steel. The new part is made from 3/16″ welded steel.
So I took a bit of time, and some very careful measurements. Didn’t want to get in a rush here, screw it up. Not like I’ve ever done THAT before. 😉
Then get out the cutting tools, and make something happen. I cut the broken bits off with a sawzall. I did some additional measurements, and cut off another 1/4″. Measure 8 times, cut twice. 🙂
After making the side plates, I bolted everything in place for a mock up. I wanted to get the pinion angle in exactly the right place. I did have to grind a smidgen off the plates before I was happy with everything.
Then back to the bench to finish welding the box. After that, added the side “wings” for lateral support. The whole thing is a bit heavy. I considered drilling some holes to make it a little lighter. But that would propagate cracks, and defeat the whole purpose. Remember, my primary goal with this car is reliability.
Here is, all pretty and ready to install. The piece on the right is what I cut off.
Like my friend Gil once said, “I’m not a great welder. But I can grind really well!”
In addition to the new brace, I finally did install the aluminum blocks instead of poly bushings. Maybe that will hold the diff in place. I used these on my Factory Five Racing roadster for more than a decade without any problems. Probably should have done that to begin with.
I installed the bolts with red Loctite and an impact wrench. But, just to be sure, I also drilled the bolt heads for safety wire.
Here she is ready to back to the track. I’m hoping I never have to mess with this again.
